1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) filters used as, for example, band-pass filters. More specifically, the present invention relates to a dual-track SAW filter including first and second acoustic channels.
2. Description of the Related Art
Various types of dual-track SAW devices including two acoustic channels have been proposed. For example, Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-77974 discloses a SAW filter including two acoustic channels which are connected in parallel. The two acoustic channels are in phase within a pass band, while they have opposite phases outside the pass band.
Also, Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-53581 discloses a SAW filter in which first and second filter tracks, defining first and second channels, are arranged on a piezoelectric substrate in parallel. In this SAW filter, an input IDT electrode of the first filter track is electrically connected in parallel with an input IDT electrode of the second filter track, and an output IDT electrode of the first filter track is electrically connected in parallel with an output IDT electrode of the second filter track. The first and second filter tracks are in phase within a pass band, while they have opposite phases outside the pass band. Furthermore, in the SAW filter according to Patent Document 2, frequencies of transmission functions (maximum value—3 dB), that is, 3 dB center frequencies of the first and second filter tracks substantially match with each other.
More specifically, the SAW filter disclosed in Patent Document 1 is used as an IF filter for PCS. In PCS, the ratio between a 30 dB bandwidth and a 3 dB bandwidth, that is, the shape factor (S.F.), is preferably about 1.8 at the maximum. On the other hand, IF filters for a W-CDMA scheme or digital satellite radio are required to have an S.F. of about 1.5 or less. That is, a curve at the vicinity of the pass band in the filter characteristic is steeper.
When the dual-track SAW filter described in Patent Document 1 is used where a steeper filter characteristic is required, a region where the first and second acoustic channels have opposite phases and the same amplitude must be ensured over a wide range of frequencies. However, when an attempt is made to realize the region of opposite phases and the same amplitude over a wide range of frequencies by main lobes of the first and second acoustic channels, the phases cannot be canceled out over a sufficiently wide range of frequencies, because inclinations of the phases of the two acoustic channels are different. That is, a large response remains outside the pass band.
In the SAW filter described in Patent Document 2, steepness of the filter characteristic of the entire filter is determined by steepness of the filter characteristic of one of the filter tracks. Also, the first and second filter tracks have opposite phases outside the pass band in order to increase attenuation outside the pass band, as in Patent Document 1. Therefore, the steepness of the filter characteristic cannot be increased to so as to be steeper than one of the filter tracks.